It is proposed that potentiometric electrodes be constructed by covalent attachment of peptidases to graphite rods, and that these be fully characterized and employed for specific detection, identification, and measurement of peptide substrates. Initial studies will employ the more readily available digestive and bacterial proteinases of various mechanistic types; the enzymes will be attached via a cyanuric chloride linkage to a functionalized graphite surface, and the resulting electrodes will be characterized as fully as possible with respect to enzyme leading and activity optimal performance conditions, concentration dependence of signal, substrate specificity, interference, and control studies. These electrodes are expected to be useful for routine analysis and chromatographic detection of peptides, and will serve as models upon which further similar studies will be based. These will employ highly specific systemic and cellular peptidase, such as rat-muscle insulin proteinase and angiotensin-convertly enzyme, as the basis for potentiometric electrodes. The latter are expected to be highly specific toward certain peptide structures, and sensitive to peptide concentrations at or below 10 to the minus eight M. Evidence is presented that such electrodes may be useful for continuos monitoring of peptide hormone concentration of peptide hormones and similar species in vivo; such capabilities, which are presently unavailable, would greatly facilitate many aspects of biomedical research and may have clinical applications as well.